The detection and analysis of vibration in machines having large rotating components has long been known and utilized as a technique to detect incipient failures. Even techniques of limited reliability were deemed desirable because of the catastrophic consequences of failures, not only in terms of the destructive potential of a failed rotating component, but also because of the economic losses resulting during the time required to repair or replace such components or entire machines.
The problem of incipient failure is a more serious one when the machine with the rotating component is an aircraft jet engine. The modern engine includes several concentrically mounted shafts, each rotating at a slightly different frequency. Each of the rotating shafts or spools can include a compressor-turbine assembly which in turn includes fan and turbine blades. In the intake stages the blades are used to compress incoming air, and in the output stages the blades drive the compressor.
Machinery having massive rotating components, such as jet aircraft engines but also including large motor generators, turbines and the like, may experience bearing failures or may, through problems of wear or accident, become unbalanced and impose unacceptable loads upon the bearing and the shaft housings.
The problem, of course, is of greatest gravity when a passenger-carrying jet airliner is involved. The large fans and turbines, which are integral parts of the jet engine, can, upon catastrophic failure, penetrate the aircraft hull and cause substantial injury to the cabin and occupants, as well as impair the air worthiness of the aircraft.
For some time, techniques have been available to monitor the vibrations of rotating machinery and to signal dangerously high vibration amplitues or, at least, signal large incremental changes over the otherwise normal patterns of vibration. Such techniques are also available to assist in the balancing of the rotating components to keep vibrations at acceptably low levels. Limiting vibration has long been deemed a factor in prolonging the life of the bearings.
While such techniques are applicable to aircraft, the environment of the modern jet engine tends to create a high "noise" level due to the sympathetic vibrations of component parts of the aircraft. When operating, the jet engine generates a broad spectrum of frequencies including harmonics which usually excite most if not all resonances. Accordingly, it is difficult to monitor the vibrations that are directly related to the main rotating components of an engine in the presence of all of the other components of "noise".
Prior art techniques have utilized filters in an attempt to isolate the vibrations attributable to the engine components, and these filtered signals are then processed to provide a quantitative display that a trained observer could interpret. The observer, noting the amplitude over a period of time can then judge if a malfunction is threatened or if one exists.
In the prior patent to Cochard, U.S. Pat. No. 4,213,114 of July 15, 1980, a system was disclosed utilizing collocated transducers which were alternately sampled. A broad-band channel is used which includes a broad-band filter whose output is integrated, rectified and, if selected, can be displayed. The integrater output is also applied to two or more narrow band channels corresponding to the co-axial shafts which have different frequencies of rotation.
A tachometer is associated with each shaft and is used to control phase-locked loop frequency multipliers whose outputs are applied to monostable circuits which control conventional analog tracking filters, the output signals of which depend only on the amplitude of the basic frequency of the input signal from the integrater.
It has been found that analog circuits in general must be designed for specific applications, and further, tend to be susceptible to noise and electrical disturbances which could adversely affect the integrity of the output signals. Further, the frequencies of interest, which range from 20 Hz to 200 Hz, are not easily accommodated in analog circuits.
According to the present invention, a system based upon a digital computer has been created which converts the analog output of a transducer, such as an accelerometer, into digital signals. The tachometer signals are easily provided as a pulse train whose frequency is related to the frequency of the rotating component of interest. Standard digital components, including counters and memory devices, are utilized to generate a sampling pulse train, whose frequency is a predetermined, integral fraction of the frequency of the rotating component, so that a suitable number of samples of the accelerometer output can be digitized to represent adequately the quantities sensed by the transducer.
A nonrecursive digital filter is created utilizing a memory in which coefficients are stored. Each digitized sample is processed through the digital filter to create a digitized output. The digital computer then converts the filter output to an RMS value which is converted in a digital to analog converter, and the resultant analog signal can be applied to drive a meter display.
Because a general purpose digital computer is employed in conjunction with memory, the apparatus can, through programming, be adapted to perform other functions. For example, vibrations at virtually any frequency of interest can be detected and displayed.
In-flight balance determinations can be made as well as trend analysis for maintenance of the rotating component. The memory can store and retain instances of extreme vibrational amplitude or other transitory events for study and later analysis. The same apparatus can be adapted for different machinery, different frequencies, and the programmable digital filter can be reprogrammed to exhibit special or different characteristics. The digital computer provides the appropriate clock frequencies for tracking and performs the desired analysis.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.